The Submerged Arc Welding Process
The Submerged Arc Welding (SAW) process is a method of electrical fusion welding performed with a concealed arc. In contrast to arc welding with welding electrodes, the arc in this case is hidden from view and burns under a blanket of slag and flux. One of the characteristic features of Submerged Arc Weld (SAW) is its high deposition rate, which essentially stems from the high current strength which is applied combined with favorable heat balance. This deposit, as is also called weld bead, is an obstacle to the inspection since it forms a protrusion that creates mechanical constraints for the water wedges positioning.
Helical Submerged Arc Welding (herein later as HSAW) is one type of SAW process and is commonly used in steel industry for many applications including joining hot rolled coil (of various widths) creating a helical seam around the finished pipe. SAW or Submerged Arc Welding means the electrode used to join the coil edges is submerged in flux to protect the weld pool from contamination. HSAW and LSAW (Longitudinal Submerged Arc Weld) pipes are used as Line Pipes for Oil and Gas transportation. HSAW is also commonly used in water transportation.
HSAW pipes are large in diameter size, ranging from 16″ to 100″ with a maximum thickness of 25 mm (1 inch).
Weld seam angles vary and are function of the coil width and pipe diameter. The range of the weld seam angles is approximately from 51 to 75 degrees. There are also different types of weld seams that require different types of inspection solutions. The weld seam angles are key-parameters for the suggested solution as it defines directly the oblique orientation of the Longitudinal and Transversal defects.
Prediction of the weld bead width and reinforcement is the subject of advanced research in metallographic and welding process; there are no simple formulae to evaluate these parameters. The integrated forming of SAW welding line can be regarded as part of the functions of conventional spiral pipe manufacturing facility. This type of manufacturing process requires an in-line inspection configuration where the pipe length is infinite and the inspection data storage needs to be synchronized with the moving saw.
Facilities with separate forming and SAW welding line with a manufacturing process requires an off-line inspection configuration where the pipe length is finite and known prior to the inspection.
Existing Test Method for Ultrasonic Inspection of Welded Pipe
For weld inspection in pipes, the cross-section to be tested is reduced to the weld seam itself and to the Heat-Affected Zones (HAZ) abutting the weld. The welding process is already automated in order to make an automated testing system worthwhile.
The common existing inspection and measurements tasks performed by a group or array of single or dual-element probes ultrasonic probes (UT probes) to inspect the defects are as follows.                i. Longitudinal defects, including internal (LID) and external (LOD) notches, through-hole defects (TDH);        ii. Longitudinal defect of mid-seam mid-wall flat-bottom hole (MWFBH);        iii. Transverse defects (internal and external notches or “T ID and T OD in short form, respectively);        iv. Lamination testing within the Heat-affected Zone (HAZ);        v. Wall Thickness Measurement in HAZ.        
Inspection standards typically require the detection of the reference defect from multiple directions. As defined in inspection standard “DNV-OSOF101 APPENDIX D” (paragraph 1311, 1312 and 1313, later as “Standard”), it is required to use opposing probe configuration (from each side of the weld). Opposing probe configuration for HSAW welds longitudinal defects are referred to as forward (FW) and backward (BW) while opposite probe configuration for HSAW welds for transversal defects are referred to as clockwise (CW) and counterclockwise (CCW).
With the conventional existing techniques using UT to inspect HSAW, each test of the above listed requires a respective and substantially accurate incidence angle. Longitudinal probe pairs, sometimes tandem probes, transverse and lamination probes are supplied. This quickly results in a testing system with a multitude of electronic channels and probes.
It is known that in order to achieve to above tasks, in existing practice using UT probes to perform the inspection, at least four groups with a total number of at least 18 are required. More should be noted that it is not just the large number of probes involved in the existing practice, each of those pairs of probes need accurate mechanical adjustment with respect of the tube diameter and thickness for the inspection to be reliable. The requirement of such constant adjustment largely impedes the production rate. Otherwise the reliability of the inspection suffers.
Since the testing mechanics has to be adjustable in accordance to the weld angle and space is rather limited when more than four probe pairs is required, a second round of weld testing mechanics and a second machine stand are often needed.
In addition, the use of UT probe pairs centered with respect to the weld allow for the detection of typical defects within the weld and also for using the through-transmission signal for constant coupling check between the two probes. If the V-transmission signal is missing or weakened, the culprit is either due to the coupling, the probe(s) or the entire system that is not working correctly. Thus, the transmission signal is constantly supervised to ensure a stable operation of the system. If the typical ultrasonic beam cannot cover the entire wall thickness, more than one probe pair has to be used.
Existing effort has been seen in patents U.S. Pat. Nos. 3,552,191, 3,868,847 and 4,131,026, each of which provides improvement to the system or method of using conventional single element or dual element UT probes to perform the inspection delineated above. US
U.S. Pat. No. 3,552,191 uses ultrasound probes to inspect some regions of an HSAW with a flat (non-curved) layout by mechanically moving the probes. Both U.S. Pat. Nos. 3,868,847 and 4,131,026 uses a series of fixed transducers (UT) in different operating modes to scan a longitudinal weld line.
U.S. Pat. No. 5,583,292 teaches the usage of phased array technology for weld line inspection. However, it does weld line inspection by measuring the wall thickness of a weld line. It does not address full-weld width inspection for all range of flaws along a helical weld line, as required in the Standard. The contents of the aforementioned U.S. patents are incorporated by reference herein.
The weld inspection requires a smooth helical pipe movement with respect to the probes, making the seam tracking an essential but difficult task.